1. Field of the Invention
The present invention relates to a magnetic storage device in which a reading circuit for reading storage information from a memory cell formed using a magnetic material is improved.
2. Description of the Related Art
In recent years, much attention has been paid to a magnetic storage device using a magneto-resistance random access memory (MRAM) cell in which a resistance value changes in accordance with storage information.
In a reading circuit which reads the signal of this type of MRAM cell, the output current of the MRAM cell is supplied to a resistor, and the output current is converted into a voltage. Then, this voltage value is compared with a reference value by a comparison circuit, to judge the storage state of the MRAM cell. The output current of the MRAM cell is a micro value of about several hundred μA. To correctly judge the storage state by the comparison circuit, the micro current value of the MRAM cell needs to be converted into a comparatively large voltage value of about several hundred mV. Therefore, a large resistance value is necessary. To realize this large resistance value, a large area is necessary, which is a problem in integration.
As a method for solving this problem, a conversion method by capacity discharge using a capacitor has been suggested (U.S. Pat. No. 6,741,490, Micron Technology, May, 2004). This method utilizes properties that the voltage changes in accordance with the amount of electric charges accumulated in the capacitor. Furthermore, the voltage value of a capacity end due to a reference current is compared with that of a capacity end due to a cell current, to read the signal of the cell. In this method, a large resistance for the current-voltage conversion can be realized with a micro capacity, to decrease the area for the voltage conversion.
However, in the reading circuit of the capacity discharge system, a complicated voltage comparison circuit for detecting the signal and a latch circuit which holds the comparison result are necessary. To constitute these circuits, a large number of transistors need to be used, which is a cause to bring about the increase of the area and the increase of power consumption.